Cytidine, N-acetyl-2′-O-[2-[(1,4-dioxopentyl)oxy]ethyl]-5′-O-[1-(diphenylmethoxy)-3 ,3,3-trimethyl-1-[(trimethylsilyl)oxy]disiloxanyl]-, 3′-[methyl bis(1-methylethyl)phosphoramidite]